Current state of the art communication systems transmit information in the form of digital signals. The digital signals consist of bipolar pulse code modulated (PCM) signal pulses wherein information is represented by a binary "1" or a binary "0" and wherein a "1" may be a positive going pulse or a negative going pulse. Typically, such PCM pulses are transmitted as Alternate Mark Inverted (AMI) pulses, in which every other pulse is inverted to avoid build-up of a D.C. level on the transmission line. The AMI pulse may be either a Return-To-Zero (RZ) or Non-Return-To-Zero (NRZ) pulse. In the RZ system, each pulse is allowed to go from a positive or negative value back to zero, whereas in the NRZ system, the pulse is not allowed to return to zero at the end of its period. The present invention is directed to PCM, AMI, RZ telephone communication from a subscriber over two-wire conductive lines, referred to as loops or wire pairs. These lines represent an existing dedicated facility created at great expense and which requires a large amount of labor and investment to maintain. If the capacity of these two-wire lines could be increased by increasing the bit rate at which information could be communicated, without degrading the signal, substantial benefits would result.
One of the factors limiting the bit rate in wire pairs is the need to manage the subscriber loop electromagnetic spectrum to allow various services to coexist in adjacent subscriber loops without creating high frequency signals which would interfere with one another.
A need exists, therefore, for a method and apparatus for increasing communication bit rates over wire pairs without exceeding predetermined frequency constraints which would result in performance degradation.